This invention relates to a tube or pipe adapted for heating fluids carried therewithin.
The excellent properties of polytetrofluoroethylene (PTFE), such as non-adhesiveness, resistance to corroision and high-temperatures, have led to use of the polymer as a pipe for heating a fluid flowing therethrough. In such an application, it is the common practice to coil an electric heater wire such as nichrome wire around the pipe of PTFE. An electric current is applied to the electric heater wire to generate heat, which is transmitted through the pipe to indirectly heat the fluid. However, such heating means has a number of disadvantages as summarized below.
(1) The wire is so high in heating density per output power at the heating wire surface that the temperature of the wire becomes too high, resulting in degradation of the PTFE pipe. PTFE is degraded at a relatively low temperature of about 260.degree. C. and melts at about 327.degree. C. Accordingly, portions or areas of the pipe in contact with the wire tend to be degraded rather than the entirety of the pipe.
(2) For the same reason mentioned above, a heat insulating material or an electric insulating material surrounding the wire will be degraded as well as the pipe per se. In other words, there is a limitation in the choice of materials which can be employed as such insulation.
(3) The heating by an electric heater wire is not satisfactory because such heating takes place in a linear and local manner, resulting in local superheating of fluid in the pipe.
(4) In order to avoid the local superheating, the wire can be closely wound. However this leads to an increase in total resistance, so that it becomes difficult to control the amount of heat. This may be avoided by changing the diameter or material of the wire, the applied voltage, etc., but these changes will also produce technical complications.
(5) When the temperature control of the heating pipe is provided in the local heating system of the type described above, special care must be taken in positioning a temperature sensor since temperature distribution within the pipe is not uniform.
(6) Use of wire of a small diameter may risk a break in the wire.